Transmission defaults into second, and my check engine light turns on?

The drive and neutral lights flash. When checking the code, it says that it is the transmission input speed sensor. If I clear the code the car drives fine until it comes back on. We have replaced both the input and output speed sensors, and we have checked for leaks or damage but nothing. The fluid is full but does need to be changed. What else could cause these issues? With trying different things to fix it the car has been driven some. Sometimes for several smaller trips with no issues and sometimes it will have the same issue several times before going around a city block.

What can we do next to try and diagnose/fix the problem?